Conventionally, there has been known that with respect to a material which possesses the metal structure such as a metal body, the strength or the ductility of the material can be enhanced by turning the metal structure into the finer grain structure using an ECAP (Equal Channel Angular Pressing) method.
In the ECAP method, as shown in FIG. 33, the metal structure is turned into the finer grain structure in such a manner that an insertion passage 200 which has a midst portion thereof bent at a desired angle is formed in a die 100, a desired metal body 300 is inserted into the insertion passage 200 by pushing so as to bend the metal body 300 along the insertion passage 200 and hence, a shearing stress is generated in the metal body 300 due to such bending, whereby the metal structure is turned into the finer grain structure due to the shearing stress. In FIG. 33, numeral 400 indicates a plunger which pushes the metal body.
In such an ECAP method, to facilitate the bending of the metal body 300 along the insertion passage 200, the deformation resistance is lowered by heating the whole metal body 300 by heating the die 100 at a given temperature. However, when the deformation resistance of the metal body 300 is largely lowered, there exists a possibility that the undesired deformation such as buckling is generated in the metal body 300 when the metal body 300 is pushed by the plunger 400 and hence, it is necessary to suppress the heating of the metal body 300 to a necessary minimum.
When the heating of the metal body 300 is suppressed, since it is necessary to push the metal body 300 by the plunger 400 with a relatively large force, there has been a drawback that the formability is poor.
Accordingly, in a method for processing a metal material and an apparatus for processing the metal material disclosed in Japanese patent laid-open-2001-321825, there has been proposed a technique in which a shearing deformation region of an insertion passage where a shearing stress is applied to a metal body is locally heated and hence, the deformation resistance of a shearing deformation portion of the metal body is reduced by heating, whereby a force which pushes the metal body using a plunger can be decreased thus enhancing the formability.
However, usually, when a portion of a metal-made die is locally heated, the whole die is heated to a given temperature due to the influence of thermal diffusion and hence, the formation of the locally heated region is difficult.
Accordingly, so long as the metal body is inserted in the insertion passage, the metal body is continuously heated at the given temperature and hence, there has been a possibility that the metal structure which is once turned into the finer grain structure by the shearing stress becomes coarse.
Further, since the ECAP method is required to use the die which is a consumable product, it is necessary to exchange the die depending on the durable condition of the die thus also giving rise to a drawback that a manufacturing cost is pushed up.
In such circumstances, recently, in an automobile industry particularly, the reduction of weight of a vehicle body or the like is desired for enhancing the mileage or for enhancing the traveling performances. Here, there exits a considerable demand for the reduction of weight by making use of a metal body which can obtain a high strength by making the metal structure finer not only with respect to high-class cars but also with respect to general-use cars. Accordingly, there exists a potential demand for a metal body which possesses a high strength or high ductility at a low cost.
Inventors of the present invention have made research and development for manufacturing various kinds of metal bodies which possess the high strength or the high ductility at a low cost by turning the metal structure into a finer grain structure and have arrived at the present invention.